In the film-to-digital transition for motion pictures, digital projection techniques are expected to meet or exceed stringent quality requirements and to provide sufficient brightness for large venue projection. One type of spatial light modulator (SLM) used for earlier digital projection apparatus is the LCD (Liquid Crystal Device). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have some advantages as spatial light modulators for high-quality digital cinema projection systems. These advantages include relatively large device size, favorable device yields and the ability to fabricate higher resolution devices, such as the 4096×2160 resolution devices available from Sony and JVC Corporations.
LCOS (Liquid Crystal On Silicon) devices are thought to be particularly promising for large-scale image projection. However, with LCD components it can be difficult to maintain the high quality demands of digital cinema, particularly with regard to color and contrast, since the high thermal load of high brightness projection affects polarization qualities of these devices. The current commercialized projectors use at least one short arc xenon lamp system that is split into three-color bands (red, green, and blue) by an optical splitter. This light is then delivered to one of three LCOS SLMs by either a polarization beam splitter cube or a wire grid polarizer. The cost and complexity of this system is hindered by inefficiencies in illumination. Because of modulator size and other factors, larger and more expensive components, including, fast optics (approximately f/2.3) are often needed to collect and deliver the light. The projection lenses, for example, can be priced at well over ten thousand dollars.
A second type of SLM that has enjoyed some success in projection solutions for multicolor digital cinema projection is the Digital Light Processor (DLP), a digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DLPs have been successfully employed in digital projection systems. DLP-based projectors demonstrate the capability to provide the necessary light throughput, contrast ratio, and color gamut for most projection applications from desktop to large cinema.
FIG. 1 shows a simplified block diagram of a projector apparatus 10 that uses DLP spatial light modulators. In large venue projection, where sufficient light levels are difficult to achieve, three color modulators are typically used so that all three color bands may be shown simultaneously. A light source 12 directs polychromatic unpolarized illumination into a prism assembly 14, such as a Philips prism, for example. Prism assembly 14 splits the polychromatic light into red, green, and blue component wavelength bands and directs each band to a corresponding spatial light modulator 20r, 20g, or 20b. The spatial light modulator has a spatial array of independently addressable micro-mirrors that selectively direct illumination light to either the projection optics or a beam dump. Prism assembly 14 then recombines the modulated light from each SLM 20r, 20g, and 20b and directs this unpolarized light to a projection lens 30 for projection onto a display screen or other suitable surface.
As an example of prism use, one conventional scheme pertains to a prism assembly and projection lens with a defined aperture to manage the illumination and separation of multiple mirror elements in two states. Others conventional adaptations of this principle for micro-mirror based modulators exist. In addition, another conventional solution involves multiple cross-prisms, one in each light modulation channel, each directing light to a fourth, combining prism.
As can be seen from the examples just cited, prism assembly 14 has been a basic component required in the design of most DLP-based digital cinema projectors. While designs using light-distribution prisms have shown some success, however, they present some significant disadvantages, including size and weight. These physical requirements are due to factors such as the needed power levels and overall range of light angles of the illumination it receives, as well as to the complexity of the prism task, separating, redirecting, and recombining light from each of its DLP devices. Still other difficulties are related to critical surface tolerances and the level of cleanliness required for light redirection using total internal reflection. For example, dirt and defects can cause improper imaging of an incorrect state of the modulated light. Problems such as these typically cause this assembly to be very expensive. Moreover, the bulk glass used for the prism, expensive and difficult to fabricate, requires the use of a short working distance of the projection lens, adding to the expense of the projection lens.
Factors such as illumination type, spatial light modulator size and the speed of the optical system, as discussed earlier, relate to etendue or, similarly, to the Lagrange invariant. As is well known in the optical arts, etendue is a measure of the amount of light that can be handled by an optical system. Potentially, the larger the etendue, the brighter the image. Numerically, etendue is proportional to the product of two factors, namely the image area and the numerical aperture. In terms of the simplified optical system represented in FIG. 2 having light source 12, optics 18, and a spatial light modulator 20, etendue is a product of the area of the light source A1 and its output angle θ1 and, in a well-matched optical system, this is equal to the product of the area of the modulator A2 and its acceptance angle θ2. For increased brightness, it is desirable to provide as much light as possible from the area of light source 12. As a general principle, the optical design is advantaged when the etendue at the light source is most closely matched to the etendue at the modulator.
Increasing the numerical aperture, for example, increases etendue so that the optical system captures more light. Similarly, increasing the source image size, so that light originates over a larger area, increases etendue. In order to utilize an increased etendue on the illumination side, the etendue must be greater than or equal to that of the illumination source. Larger image sizes, however, typically result in a more costly system. This is especially true of devices such as LCOS and DLP components, where the silicon substrate and defect potential increase with size. As a general rule, increased etendue results in a more complex and costly optical design. Using a conventional approach, for example, lens components in the optical system must be designed for large etendue. The source image area for the light that must be converged through system optics is the sum of the combined areas of the spatial light modulators in red, green, and blue light paths; notably, this is three times the area of the final multicolor image formed. That is, for the configuration disclosed in such a conventional approach, optical components handle a sizable image area, therefore a high etendue, since red, green, and blue color paths are separate and must be optically converged. Moreover, although a configuration handles light from three times the area of the final multicolor image formed, this configuration does not afford any benefit of increased brightness, since each color path contains only one-third of the total light level.
Optical efficiency improves when the etendue of the light source is well-matched to the etendue of the spatial light modulator. Poorly matched etendue means that the optical system is either light-starved, unable to provide sufficient light to the spatial light modulators, or inefficient, effectively discarding a substantial portion of the light that is generated for modulation.
The goal of providing sufficient brightness for digital cinema applications at an acceptable system cost has eluded designers of both LCD and DLP systems. LCD-based systems have been compromised by the requirement for polarized light, reducing efficiency and increasing etendue, even where polarization recovery techniques are used. DLP device designs, not requiring polarized light, have proven to be somewhat more efficient, but still require expensive, short lived lamps and costly optical engines, making them too expensive to compete against conventional cinema projection equipment.
In order to compete with conventional high-end film-based projection systems and provide what has been termed electronic or digital cinema, digital projectors must be capable of achieving brightness levels on the order of 10,000 lumens, projected onto screen sizes on the order of 40 feet in diagonal. The range of screens requires anywhere from 5,000 lumens to upwards of 40,000 lumens. In addition to this demanding brightness requirement, these projectors must also deliver high resolution (2048×1080 pixels) and provide around 2000:1 contrast and a wide color gamut.
While current commercialized digital cinema projector designs have proved to be capable of this level of performance. However, high equipment cost and operational costs have been obstacles in the transition to digital. Projection apparatus that meet these requirements typically cost in excess of $50,000 each and utilize high wattage Xenon arc lamps that need replacement at intervals between 500-2000 hours, with typical replacement cost often exceeding $1000. The large etendue of the Xenon lamp has considerable impact on cost and complexity, since it necessitates relatively fast optics to collect and project light from these sources.
One drawback common to both DLP and LCOS LCD spatial light modulators (SLM) has been their limited ability to use solid-state light sources, particularly laser sources. Although they are advantaged over other types of light sources with regard to relative spectral purity and potentially high brightness levels, solid-state light sources require different approaches in order to use these advantages effectively. Conventional methods and devices for conditioning, redirecting, and combining light from color sources, used with earlier digital projector designs, can constrain how well laser light sources are used.
Solid-state lasers promise improvements in etendue, longevity, and overall spectral and brightness stability but, until recently, have not been able to deliver visible light at sufficient levels and at costs acceptable for digital cinema. In a more recent development, laser arrays have been commercialized and show some promise as potential light sources. However, brightness itself is not yet high enough; the combined light from as many as 9 individual arrays is needed in order to provide the necessary brightness for each color.
Laser arrays of particular interest for projection applications include various types of VCSEL arrays, including VECSEL (Vertical Extended Cavity Surface-Emitting Laser) and NECSEL (Novalux Extended Cavity Surface-Emitting Laser) devices from Arasor, Sunnyvale, Calif. However, conventional solutions using these devices have been prone to a number of problems. One limitation relates to device yields. Due largely to heat and packaging problems for critical components, the commercialized VECSEL array is extended in length, but limited in height; typically, a VECSEL array has only two rows of emitting components. The use of more than two rows tends to increase yield difficulties dramatically. In addition, conventional VECSEL designs are prone to difficulties with power connection and heat sinking. These lasers are of high power; for example, a single row laser device, frequency doubled into a two row device by Arasor, produces over 3 W of usable light. Thus, there can be significant current requirements and heat load from the unused current. Lifetime and beam quality is highly dependent upon stable temperature maintenance.
Coupling of the laser sources to the projection system presents another difficulty that is not adequately addressed using conventional approaches. For example, using Arasor NESEL lasers, approximately nine 2 row by 24 laser arrays are required for each color in order to approximate the 10,000 lumen requirement of most theatres. It is desirable to separate these sources, as well as the electronic delivery and connection and the associated heat from the main thermally sensitive optical system to allow optimal performance of the projection engine. Other laser sources are possible, such as conventional edge emitting laser diodes. However, these are more difficult to package in array form and traditionally have a shorter lifetime at higher brightness levels.
Thus, it can be seen that the challenge of providing lower cost optical system having cinema or near-cinema performance and brightness has not been met using conventional approaches. There is a need for an illumination solution that enables laser light to illuminate micro-mirror spatial light modulators in a cost effective simple manner at the brightness levels needed for high-end projection systems.